How to create a gradient texture from one material to another (opaque to clear)?
All you need to do is use a Gradient Texture (see how to create it procedurally here: How to create a gradient texture?) as a mask for the transition between two materials.


Set up the nodes as pictured below. Try to experiment with a Mapping Node settings (adjust the Location, Rotation and Scale parameters) to achieve a desired effect.


To have a better control between materials transition add a Color Ramp node and experiment with its slider.